Therefore, one can assume the presence of a system that facilitates detection of a change, orientation to the new event and a subsequent behavioral response. Studies using functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) have demonstrated change-specific cortical activation in the auditory, visual and somatosensory systems. The quick detection of a change in the sensory environment is very important to survival. The STG works as a real-time sensory gate open to a new event. One presentation of a sound is enough to shape a memory trace for comparison with a subsequent physically different sound and elicits change-related cortical responses in the STG. Whereas activation in the planum temporale did not differ among events, activation in the superior temporal gyrus (STG) was clearly greater for the different events (1D, 2D, 3D) than the same event (1S and 2S). Sounds were grouped to five events regardless of their frequency, 1D, 2D, and 3D (a sound preceded by one, two, or three different sounds), and 1S and 2S (a sound preceded by one or two same sounds). Under a paradigm where two pure sounds 300 ms in duration and 800 or 840 Hz in frequency were presented in a specific order at an even probability, cortical responses to each sound were measured with magnetoencephalograms. To test this hypothesis, we examined whether the single presentation of a sound is enough to elicit a change-related cortical response, and therefore, shape a memory trace enough to separate a subsequent stimulus. Given that sensory memory monitors current sensory status and works to pick-up changes in real-time, any change detected by this system should evoke a change-related cortical response. The process should relate closely to memory since it requires that the brain separate a new stimulus from an ongoing background or past event. The rapid detection of sensory change is important to survival.
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